Hopes for a peaceful end to the Syrian crisis were fading rapidly on Sunday after President Bashar al-Assad refused to meet Tuesday's deadline for withdrawing his forces from flashpoints across the country without guarantees from the opposition to stop fighting first.

Kofi Annan, the former UN secretary-general, had called for a pullout to be completed by Tuesday morning with a ceasefire taking effect 48 hours later as the first stage in a six-point international plan for political negotiations between the Assad regime and its opponents.

Amid evidence of continuing government operations, Syria's foreign ministry said it was demanding written guarantees that "armed terrorist groups" would lay down their weapons.

The Free Syrian Army, the opposition's main fighting wing, quickly blamed the government for undermining the deal.

"The regime will not implement this plan," the FSA commander Colonel Riad al-As'ad, told Reuters from his Turkish headquarters. As'ad and other officers said the rebels had promised to stop fighting if the government did the same.

BBC Arabic reported that the FSA had refused to give any guarantees.

But the continuing bloodshed, deep mistrust after 13 months of the uprising, an estimated 9,000-10,000 dead and the fragmented nature of the anti-Assad forces meant that any guarantees were unlikely to be forthcoming and meaningless if made.

On the ground, the Syrian Revolution General Commission reported 32 dead on Sunday. Signs of intensifying military activity included YouTube footage of Mig29 fighter jets flying over Aleppo, military helicopters firing rockets and gruesome images of shabiha militamen in Deraa cursing and manhandling the bodies of dead rebels. None could be independently authenticated.

Opposition activists poured scorn on Assad's move but argued it was entirely consistent with his strategy of buying time and exploiting the divisions of opponents at home and abroad. But many also blamed the former UN secretary-general for being naive and gullible enough to believe Assad had any intention of compromising. "The Syrian regime has just given Annan the finger," said one.

Scepticism about Assad's readiness genuinely to implement Annan's plan was rife from the start, but the western governments who dominate the Friends of Syria group had argued that it was the "only game in town" given Russian support for Assad and the lack of appetite for outside intervention.

"This is a totally predictable move," said Emile Hokayem of the International Institute for Strategic Studies. "By accepting Annan's plan, Assad wanted to demand - and get - reciprocity to put pressure on a fragmented opposition. Russia will support this demand and the Friends of Syria have painted themselves into a corner. Can they really say no at this point? And Assad thinks that the moment a senior diplomat or official knocks at his door, that he's back in the game."

Annan said in a statement: "I am shocked by recent reports of a surge in violence and atrocities in several towns and villages in Syria, resulting in alarming levels of casualties, refugees and displaced persons, in violation of assurances given to me."

Saturday saw more deadly attacks, with between 74 and 160 people reported killed under bombardment in the provinces of Idlib and Aleppo. Nearly 70 were killed in al-Latmaneh, near Hama. The figures could not be confirmed independently because of restrictions on the media.

Jihad Makdissi, spokesman for the Syrian foreign ministry, told Syrian state television on Sunday that the government would not repeat what happened last December, when Arab League monitors were briefly deployed.

"It committed to the exit of its armed forces from the cities and surrounding areas, then the armed terrorist groups took advantage to arm its members and conduct all forms of terrorism," Makdissi said.

The regime's response suggested it was seeking a rebel surrender rather than a workable truce as a prelude to negotiations. Syrian and foreign analysts had argued that Assad could not implement the plan because it would trigger massive countrywide demonstrations that he would not be able to contain.

Last Thursday, a UN statement raised the possibility of "further steps" if Damascus failed to implement Annan's plan. But Russia and China have so far resisted any moves to condemn let alone punish Assad.

Underlining mounting concern about the crisis, Pope Benedict XVI implored Damascus to heed demands to end the bloodshed. "Particularly in Syria, may there be an end to bloodshed and an immediate commitment to the path of respect, dialogue and reconciliation, as called for by the international community," Benedict said in his traditional Easter speech.

The latest Syrian offensive has sent thousands of refugees surging into Turkey, which is now braced for a further influx.

"We are taking measures for this, though we will not close the gates," said Recep Tayip Erdogan, its prime minister. "The UN, however, has to toughen its stance."